Relación estructura-propiedades de films de nanocompuestos de pla

Abstract

The development of the present Ph.D. thesis implied the preparation of PLA and nanoclay reinforced PLA films as well as the application of different thermo-mechanical treatments. Two commercial grade PLA matrices were used, and a commercial grade of organomodified montmorillonite clay (o-MMT) was chosen. The mixing of the o-MMT and the PLA was performed using a twin-screw extruder. Films were obtained using two different processing methods: single-screw extrusion and twin-screw extrusion followed by a calendaring process. In the films, different PLA matrices were used as well as several o-MMT concentrations. In some cases a nucleating agent was added. A de-aging thermal treatment was applied to the films, allowing its study in two different states: de-aged and aged. The film¿s behavior was studied during and after thermoforming process using: (i) tensile tests at high temperatures and different deformation speeds, and (ii) sample uniaxial orientation of the material and its later characterization. Film characterization included: (i) nanocomposites¿ morphological analysis (ii) image analysis (iii) molecular size determination of the matrix (iv) thermal behavior analysis (v) mechanical properties determination in tensile mode, and (vi) fracture parameters determination using the EWF technique. Morphological analysis revealed that the nanocomposites presented intercalated structures, although exfoliated laminates and agglomerated particles were observed. FIB-SEM showed that the extrusion-calendaring process generated an anisotropic morphology, which became evident in the PLA/o-MMT films¿ mechanical and fracture behavior. The processing reduced the average molecular weight in the matrices of all the prepared films. Nevertheless, this reduction was more significant in the nanocomposite films than in neat PLA films, and increased with the clay content increase. On the other hand, the de-aging treatment promoted a fragile-ductile transition, which allowed the application of the EWF technique, enabling to correlate the parameters obtained with the thermal, mechanical and morphological characterization. Mechanical parameters in tensile mode were affected with 2.5 wt % clay content. Nevertheless, the presence of clay produced changes in the deformation mechanism, evidenced by a stresswhitening phenomenon due to the formation of micro defects as a product of o-MMT decohesion. The PLA fracture toughness increased with the thermal de-aging treatment. The addition of o- MMT increased the toughness of the aged films, due to the development of energy dissipation mechanisms such as de-cohesion and micro-void growth. This fracture toughness increase is ¿masked¿ in the de-aged films due to the elevated fracture toughness of the matrix. Finally, uniaxial tensile tests performed at high temperatures and different crosshead speeds produced a mesomorphic phase in the PLA matrix, identified with WAXS and FT-IR. The characterization of previously oriented samples revealed the influence of the thermoforming process on the mechanical and thermo-mechanical behavior, relating these variations with the induced morphology by stretching. These results allowed establishing the optimal matrixadditive- processing conditions for the fabrication of thermo-mechanical resistant PLA products through thermoforming.

El desarrollo de la presente tesis doctoral implicó la preparación de films de PLA y PLA reforzado con arcilla, así como la aplicación de diferentes tratamientos termomecánicos a los mismos. Las matrices utilizadas fueron dos grados comerciales de PLA, mientras que la arcilla elegida fue una montmorillonita comercial organomodificada (o-MMT). El mezclado de la o-MMT con el PLA se realizó mediante extrusión doble husillo. Los films se obtuvieron utilizando dos vías de procesamiento diferente: extrusión monohusillo y extrusión doble husillo, ambas seguidas de un calandrado. En los films se varió la matriz, la concentración de o-MMT y, en algunos casos, se adicionó un agente nucleante. A los films se les aplicó un tratamiento térmico de rejuvenecimiento, lo que permitió su estudio en dos estados diferentes: rejuvenecido y envejecido. El comportamiento de los films durante y después de un proceso de termoconformado se estudió mediante: (i) ensayos de tracción a elevada temperatura y diferentes velocidades de deformación y (ii) orientación uniaxial de muestras de los materiales y su posterior caracterización. La caracterización de los films incluyó: (i) análisis de la morfología de los nanocompuestos (ii) análisis de imágenes (iii) determinación del tamaño molecular de la matriz de PLA, (iv) análisis del comportamiento térmico (v) determinación de las propiedades mecánicas a tracción y (vi) determinación de los parámetros a fractura mediante la aplicación de la técnica de EWF. El análisis morfológico reveló que los nanocompuestos presentaron estructuras intercaladas, aunque también se observaron laminillas exfoliadas y partículas aglomeradas. La observación por microscopia FIB-SEM demostró que durante el proceso de extrusión-calandrado se generó una morfología anisotrópica, lo que se evidenció en el comportamiento mecánico y a fractura de los films de PLA/o-MMT.